let VS var VS const

In the release of ES6 (ECMAScript 2015) a new way of declaring variables in JavaScript has been introduced. Actually 2 new ways. let and const. With these two, we now have 3 different ways to declare variables. var, let, const

What are the differences? The main difference is the scoping 

• let
  Block scope
  
  
• const
  Block scope
  
  
• var
  Function scope

In nutshell, block scope is the code that is enclosed in curly braces

let

In the code above we are defining the variable num two times. Lines 1 and 3. What we will see in the console.log of lines 4 and 7?

Well, in line 4 we will see 10 and in line 7 we will see 50. Why is that?
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Although we have the same variable name, the scope is different. The scope of the variable in line 1 is the global while the other's scope is between lines 2 and 5. 

const

The variables that are declared with const have the same behavior like we declare them with let. They are Blocked Scope too.
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So, what's the difference then? The difference is that the variables are constant and their value won't change during the application execution. 
If we try to change the value of a constant variable, a run-time error will be thrown (const_scope_2.js)

const variables are not immutable

It's a common misconception that since we can not change the value of a variable, then we have an immutability. Well, no! We are not allowed to change the value of a variable by assigning a new object (line 10), but we are allowed to change the value of a property of this object (line 6)

var

Declaring variables with var is error prone as it is easier the value to be changed on any internal scope